Objective

Two-dimensional crystalline materials exhibit exceptional physical properties and offer fascinating potential as fundamental building blocks for future two-dimensional electronic and optoelectronic devices. Transition metal dichalcogenides (TMDCs) are of particular interest as they show a variety of many-body phenomena and correlation effects. Key properties are: i) additional internal degrees of freedom of the electrons, described as valley pseudospin and layer pseudospin, ii) electronic many-body effects like strongly-bound excitons and trions, and iii) electron-lattice correlations like polarons. While these phenomena represent intriguing fundamental solid state physics problems, they are of great practical importance in view of the envisioned nanoscopic devices based on two-dimensional materials.

The experimental research project FLATLAND will address the exotic spin-valley-layer correlations in few-layer thick TMDC crystals and TMDC-based heterostructures. The latter comprise other 2D materials, organic crystals, metals and phase change materials as second constituent. Microscopic coupling and correlation effects, both within pure materials as well as across the interface of heterostructures, will be accessed by time- and angle-resolved extreme ultraviolet-photoelectron spectroscopy, femtosecond electron diffraction, and time-resolved optical spectroscopies. The project promises unprecedented insight into the microscopic coupling mechanisms governing the performance of van der Waals-bonded devices.

The emergence of atomically flat two-dimensional crystals opened novel perspectives for condensed matter physics: the fabrication of functional devices from building blocks as thin as a single atom as well as the fabrication of new materials by stacking fundamentally different materials into arbitrary order. These two scenarios represent the device physics and material science perspectives on van der Waals-bonded heterostructures. From both perspectives, a microscopic understanding of the fundamental interactions between electrons, lattice and spins in 2D materials and heterostructures is desired. FLATLAND addresses these phenomena by developing and applying ultrafast techniques, which allow to observe the temporal evolution of non-equilibrium states on the level of single quantum states. This is achieved by a combination of different ultrafast techniques, namely time- and angle-resolved photoemission spectroscopy (trARPES), femtosecond electron diffraction (FED) and femtosecond point-projection electron microscopy (fsPPM). The experimental data achieved with these techniques can be compared to electronic and atomic structure calculations and provides benchmarks for validating computational approaches as well as for designing functional van der Waals heterostructures.

Choosing WSe2 as a model system, we obtained a microscopic understanding of the nature of excited states in this prototypical transition metal dichalcogenide (TMDC) semiconductor. This includes the spin-, valley- and layer-polarisation of bright excitons, their binding energy as well as their interaction strength with the lattice, and the size of the exciton wavefunctions. The accompanying investigation of the lattice excitations generated by the interaction of the excitons with the atomic structure yields a comprehensive picture of the non-equilibrium states in TMDC materials.

We established FED as a methodology for investigating the flow of energy through electrons and vibrations in nanoscale heterostructures. In addition, we pioneer utilizing the inelastic scattering signal for retrieving momentum-resolved information of transient phonon distributions.

In parallel, we developed four-dimensional photoemission spectroscopy, which provides a view of the electronic structure resolved in energy, time and both momentum directions. Based on this, we established excited stated mapping as an extension of the band mapping concept with ARPES to excited / non-equilibrium states. The attached image shows a snapshot of the energy and momentum-distribution of excitons in WSe2 tens of femtosecond after the creation of the exciton with a short laser pulse.A central aspect of this progress is the development of data analytics tools, which we share with the community.

We developed four-dimensional trARPES as new technique providing an unprecedented view of the electronic structure of crystals in transient non-equilibrium states. Based on this technique, we study the momentum- and energy-distribution of excitons in TMDC materials with approximately 10 femtosecond temporal resolution, which provides basic information of the exciton’s properties like exciton binding energy, the exciton self-energy and the real-space distribution of the exciton wave function. In contrast to optical techniques, our approach is sensitive to optically bright as well as dark excitonic states. Future work will extend this approach to interfacial excitons and hybrid excitons, where the exciton wave functions extend over qualitatively different materials like inorganic semiconductors and molecules. We anticipate recording a movie of the birth and evolution of excited states in molecular crystals and of excitons in van der Waals heterostructures.

The response of the atomic structure to the presence of excitons is investigated by FED. We established femtosecond inelastic electronic scattering as a technique to obtain momentum-resolved information of lattice excitations. This approach is currently applied to van der Waals heterostructures in order to retrieve the pathways of interfacial charge and energy transfer and to identify the dominating interfacial interactions.

Snapshot of the excited state distribution in momentum space of a 2D semiconductor.